This invention relates generally to the field of vehicle steering systems, and more particularly relates to an electric power assist steering system having an electric motor flexibly coupled to a vehicle steering system.
A Typical Steering System
A typical steering system for a motor vehicle is illustrated in FIG. 1. The steering system 1 has rotating steering wheel 2 in the passenger compartment of the vehicle mounted to steering column 3 that is operatively connected to wheels 4 via steering assembly 5. In order to reduce the amount of driver effort (i.e., torque) that is required to rotate the steering wheel, many steering systems include a power-assisted actuator. The actuator assists the operator with rotation of the steering wheel to overcome opposing forces such as road load forces on the road wheels and friction forces in the steering assembly. The amount of power assistance generally varies depending on the speed of the vehicle and the amount of effort applied by the vehicle operator to the steering wheel. Conventional power assist steering systems typically employ either hydraulic power assist or electric power assist mechanisms. Electric power assist mechanisms are being used in an increasing number of vehicles due to their reduced size and higher energy efficiency than hydraulic mechanisms.
Electric Power Assist Systems
An electric power assist steering (EPAS) system employs an electric motor for applying a controlled amount of torque to the steering assembly to assist the operator with rotation of the steering wheel. For example, the system illustrated in FIG. 1 includes electric motor 6 for power assist, and controller 7. The steering assembly may be a rack and pinion type that converts angular rotation of the steering wheel into a sliding motion of a rack to steer the wheels. The rack interacts with teeth on an assist pinion that is driven by the output shaft of motor 6 in response to signals from controller 7. The signals from controller 7 are designed to provide a relatively constant torque at the driver pinion.
An example of an EPAS rack and pinion assembly 10 is illustrated in FIG. 2. Inner tie rods 12 are connected to a rack and pinion mechanism contained within housing 14. Gear box 16 contains a gear reduction mechanism for the assist pinion. Electric motor 18 is rigidly mounted to gear box 16 to power the assist pinion via the gear reduction mechanism. The motor output shaft directly connects to an input shaft, which may be implemented as a worm gear shaft, in the gear reduction mechanism. A driver pinion torque sensor, as well as various other sensors, may also be included, but the driver pinion and sensors are not shown to simplify the present description. The measured torque at the driver pinion serves as an approximation of the input torque applied to the steering wheel by the vehicle operator and is commonly used to determine the amount of torque assist to be provided by the electric motor to the assist pinion. Further information about electric power assist steering systems can be found in various patents and literature references, including but not limited to U.S. Pat. No. 5,743,352, to Miller et al., and U.S. Pat. No. 6,250,419, to Chabaan et al., both of which are incorporated by reference as if reproduced in full herein.
Concerns over fuel efficiency have led to the production of smaller vehicles and/or vehicles with more aerodynamic shapes to reduce wind resistance. Due to limitations on reducing the size of the passenger compartment and concerns about passenger compartment comfort, the size of vehicle engine compartments has been reduced and their shape varied to accommodate smaller vehicle sizes and/or new vehicle body designs. The demand for more features while maintaining or increasing vehicle performance have led to an increasing number of components in smaller vehicle engine compartments which have various shapes.
An electric power assist steering system offers variable assist capabilities, more efficient energy consumption, reduced mechanism complexity, increased reliability, and responsive on-demand steering assist, as well as other advantages. Conventional steering systems and components are available from TRW, having facilities in Livonia Mich., USA, Delphi Automotive Systems, having facilities in Saginaw, Mich., USA, and NSK Ltd., having offices in Tokyo, Japan. However, the electric motor increases the size of the system, and rigid attachment of the electric motor to the rack and pinion assembly leaves little flexibility for more efficient engine compartment design and component placement. For example, the typical steering gear has a length of about 1520 mm, inclusive of the tie rods, while a typical power steering motor has a length of at least about 150 mm and a diameter of at least about 100 mm. A conventional power steering system constructed in this manner makes an unwieldy combination. Further, the bulky projection created by the motor rigidly mounted to the assembly makes it more difficult to work on, install, or remove the engine, steering system or other vehicle components in the engine or power source compartment.
As used herein, engine compartment shall refer to the vehicle compartment for an internal combustion engine power source, hybrid internal combustion engine with electric motor power source, or other vehicle power source type.
Accordingly, it is desired to provide an electric power assist steering system that increases the engine compartment utilization efficiency while also increasing the ease of repair, installation, and removal of engine, steering system and other vehicle components in the engine compartment.
In accordance with the teachings of the present invention, a steering system and method of installing a power assist steering assembly in a vehicle are disclosed. According to one aspect of the present invention, an electric power assist steering system is provided in which an electric motor is operatively engaged via a flexible coupling with the remainder of the steering system for supplying torque assist. In another aspect, a motor for power assist steering systems is disclosed having a rotating output shaft and a flexible shaft connected thereto for transferring power. A method of installing a steering system in a vehicle is also disclosed wherein the electric motor is installed independently of and then flexibly coupled to the remaining steering system components. The electric motor output shaft is located at a remote location from the pinion shaft or input shaft of the pinion gear reduction mechanism. The steering system, motor, and method of the present invention provide for greater flexibility in engine compartment design and component placement efficiency, and facilitate repair, installation, and removal of engine and steering system components.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. It is to be understood that both the preceding summary and the detailed description that follows are intended merely to be exemplary and to further explain the invention claimed. The invention may be better understood by reference to the following detailed description read in conjunction with the accompanying drawings.